You can checkout the source code here: 

If you are using the source code, you will need python, pygame, and an svn program. On windows, try TortoiseSVN.

We have an executable, and it's probably broken!
1. Download Gmail Adventures here.
2. The zip file has one folder in it called "dist."
3. Unzip the file.
4. Open the "dist" folder and double-click gmadventure.exe to start the "game."